Method of making cohesive carbon assembly and its applications

ABSTRACT

Cohesive carbon assemblies are prepared by obtaining a functionalized carbon starting material in the form of powder, particles, flakes, loose agglomerates, aqueous wet cake, or aqueous slurry, dispersing the carbon in water by mechanical agitation and/or refluxing, and substantially removing the water, typically by evaporation, whereby the cohesive assembly of carbon is formed. The method is suitable for preparing free-standing, monolithic assemblies of carbon nanotubes in the form of films, wafers, discs, fiber, or wire, having high carbon packing density and low electrical resistivity. The method is also suitable for preparing substrates coated with an adherent cohesive carbon assembly. The assemblies have various potential applications, such as electrodes or current collectors in electrochemical capacitors, fuel cells, and batteries, or as transparent conductors, conductive inks, pastes, and coatings.

This application claims priority to U.S. Provisional Application No.61/523,125, filed Aug. 12, 2011; the content of which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

This invention relates to a cohesive assembly of carbon, and to methodsfor preparing a cohesive assembly of carbon, in which the startingcarbon materials, under certain prescribed conditions, self-assembleinto a disc, wafer, film, fiber, wire, or other object of a desiredshape. In preferred embodiments, the carbon assembly prepared by theinvented method comprises carbon nanotubes. The prepared assembly showsgood mechanical strength and integrity, high carbon packing density,high surface area, and low electrical resistivity, and has variouspotential applications such as in electrical power storage andconductive coatings and films. The cohesive assembly of carbon isespecially useful as an electrode or a current collector for anelectrochemical capacitor, fuel cell, or battery. Furthermore, themethod of making the assembly is environmentally friendly andcost-effective, as the raw materials consist of only the carbon startingmaterial and water, with no toxic, corrosive, or otherwise harmfulcomponents.

BACKGROUND

Assemblies of carbon, derived from a variety of carbon sources, have amultitude of current and anticipated commercial, industrial, andhigh-technology applications. For example, activated charcoal oractivated carbon, which is usually in the form of loose powder,particles, or irregular agglomerates, has a variety of uses infiltration and catalyst support. This material has also recently beenapplied to energy storage applications, as an ionic exchange medium orcapacitor electrode material. Graphite in its various forms has numeroususes, for example, as refractory material, in brake linings, and aselectrodes in electric arc furnaces. Intercalated graphite and expandedgraphite have been studied for use as fire retardants and hightemperature applications. These cohesive carbon assemblies have manydesirable properties such as resistance to chemical attack, resistanceto high temperatures, and high surface area in the case of activatedcarbon, and electrical conductivity and lubricity in the case ofgraphite. However, these materials typically require a binder or matrixmaterial to form them into an assembly of a desired shape and size,having good mechanical strength and integrity.

More recently, assemblies of carbon nanotubes (CNTs) in various formshave attracted much attention and are being explored and developed fordiverse applications. Such assemblies have been referred to in theliterature as “buckypaper” or “buckydiscs”. For example, Dharap et al in“Nanotube film based on single-wall carbon nanotubes for strainsensing”, Nanotechnology 15 (2004), pp. 379-382, investigate the use ofisotropic films of randomly oriented CNTs as mechanical strain sensors.Cao et al, in “Random networks and aligned arrays of single-walledcarbon nanotubes for electronic device applications,” Nano Research 1, 4(2008), pp. 259-272, discuss the use of random networks or alignedarrays of CNTs as thin-film transistors. Ma et al, in “Methods of makingcarbide and oxycarbide containing catalysts,” U.S. Pat. No. 7,576,027B2, disclose catalyst supports for fluid phase chemical reactions madefrom randomly entangled CNT aggregates. And Liu et al, in“Electrochemical capacitor with carbon nanotubes,” U.S. PatentApplication Publication US 2009/0116171 A1, disclose electrolyticcapacitors having electrodes made from free-standing CNT films.

Smalley et al in “Method for producing self-assembled objects comprisingsingle-wall carbon nanotubes and compositions thereof,” U.S. Pat. No.7,048,999 B2, disclose CNT assemblies formed by a complex process of CNTend-cap removal and derivatization. The buckypaper disclosed therein isa loosely assembled CNT felt or mat that is supported on a substrate.Other structures disclosed therein such as molecular arrays andself-assembled monolayers are described as requiring a substrate ormatrix material such as a resin, metal, ceramic, or cermet. Furthermore,the self-assembled structures disclosed therein comprise functionalagents to bond the CNTs together, which may adversely affect thestructures' electrical properties.

Tohji et al in “Carbon nanotubes aggregate, method for forming same, andbiocompatible material,” U.S. Patent Application Publication US2007/0209093 A1, disclose a method for CNT aggregate formation involvingexposure to fluorine gas followed by sintering at high temperature andpressure. The aggregates are characterized as being fragile.

Liu et al in US 2009/0116171 A1, and Hata et al in “Aligned carbonnanotube bulk aggregates, process for production of the same and usesthereof,” U.S. Patent Application Publication US 2009/0272935 A1,disclose methods for preparing CNT assemblies that require the use ofCNT forests grown by CVD processes on a substrate. These methods involvea sequence of solvent washing, pressing, and/or drying steps and arelimited to the scale of the starting CNT forest. Furthermore, theseassemblies are characterized by a predominant orientation or alignmentof the CNTs, which imparts the assembly with anisotropic and largelyunidirectional properties.

Whitby et al in “Geometric control and tuneable pore size distributionof buckypaper and bucky discs,” Carbon 46 (2008) pp. 949-956, disclose afrit compression method for forming CNT assemblies, which also requireshigh pressures. Also, the CNTs are not uniformly distributed within theassemblies, and the assemblies have large macropores and very highporosity (>80%).

A method to form a solution of single-walled CNTs in sulfuricsuper-acids is disclosed by Davis et al in “Phase Behavior and Rheologyof SWNTs in Superacids,” Macromolecules 37 (2004) pp. 154-160. A methodis also disclosed to produce an entangled mat of CNT ropes by quenchingin ether and filtering.

R. Signorelli et al in “High Energy and Power Density NanotubeUltracapacitor Design, Modeling, Testing and Predicted Performance,”presented at The 19th International Seminar on Double Layer Capacitorsand Hybrid Energy Storage Devices (Dec. 7-9, 2009, Deerfield Beach,Fla., USA), and in “Electrochemical Double-Layer Capacitors Using CarbonNanotube Electrode Structures,” Proceedings of the IEEE 97, 11(2009),pp. 1837-1847, disclose vertically aligned single-walled CNT (SWCNT) andmulti-walled CNT (MWCNT) “forest”-type assemblies intended for use asbinder-free electrodes. These assemblies, however, show low bulk densityof 0.45 g/cm³ or less (0.1 g/cm³ in the case of SWCNT), requiring animpractically high volume of material for adequate capacitorperformance. Scalability of these CNT forests for manufacturing purposesis questionable, and they have inferior mechanical properties for use ascurrent collectors.

A similar forest-type assembly produced from double-walled CNT (DWCNT),intended for use as a capacitor electrode, is disclosed by T. Asari in“Electric Double-Layer Capacitor Using Carbon Nanotubes Grown Directlyon Aluminum”, presented at ICAC2010, The 2010 International Conferenceon Advanced Capacitors (May 31-Jun. 2, 2010, Kyoto, Japan). Thisassembly has similar drawbacks as that of Signorelli; namely, lowdensity, non-scalability, and inferior mechanical properties.

A. Izadi-Najafabadi et al, in “Extracting the Full Potential ofSingle-Walled Carbon Nanotubes as Durable Supercapacitor ElectrodesOperable at 4 V with High Power and Energy Density,” in AdvancedMaterials, n/a. doi: 10.1002/adma.200904349 (Published on-line June 18,2010), describe a capacitor electrode based on a high-purity SWCNTforest processed into a binder-free assembly. This assembly showsattractive electronic performance characteristics as an electrode whentested under laboratory conditions. However, a sealed capacitor devicecould not be produced using this assembly due to excessive swelling whenimpregnated with the liquid electrolyte, indicating that the assemblyhad inferior mechanical strength and integrity.

There is interest in applying CNT technology to electrochemicaldouble-layer capacitors (EDLC), sometimes referred to as“supercapacitors” or “ultracapacitors”. This capacitor type has powerdensity somewhat lower than, but nearly approaching, that of standardcapacitors, but much higher energy density, approaching that of standardbatteries. EDLCs have many applications in consumer electronics, and areattractive for use in hybrid gas-electric vehicles and all-electricvehicles. Activated carbon is the most common material currently used aselectrodes in EDLCs. However, its performance may be reaching itstechnological limit and materials capable of higher energy and powerdensities are desired, especially for vehicle applications.

Lithium-ion is one battery type of particular interest for applicationof carbon nanotubes. Modern Li-ion batteries typically comprise acarbon-based anode, a cathode comprising an oxide such as LiCoO₂,LiFePO₄, LiNiCoAlO₂, or the like, and an electrolyte comprising alithium salt in an organic solvent. Li-ion batteries are commonly usedin consumer electronics, and are attractive for use in hybridgas-electric and all-electric vehicles. However, improvements in batteryperformance are needed for widespread vehicle application. Specifically,increased energy density, power density, lighter weight, and betterreliability are desirable. Particularly attractive are thinner and/orlighter electrode materials having lower electrical resistance, moreefficient ion transfer capability, and sufficient mechanical strengthfor battery use.

In a standard fuel cell, hydrogen is combined with oxygen to generateelectric current and water as a by-product. One fuel cell type ofcurrent high interest is the proton exchange membrane or polymerelectrolyte membrane (PEM) fuel cell. This design comprises a membraneelectrode assembly (MEA), which in turn comprises a center protonexchange membrane (PEM), and an electrode on either side of the PEM.Each electrode comprises a catalyst layer and a gas diffusion layer(GDL). The catalyst layer is typically comprised of fine metal particlesor powder (platinum for the anode, often nickel for the cathode) on aporous support material such as pressed carbon black. The GDL layer,which contacts the metallic current collector on the face opposite thecatalyst layer, is usually comprised of carbon paper or carbon cloth. Asin the case of Li-ion batteries, improvements in PEM fuel cellperformance are also needed for widespread application, especially invehicles. Stronger and more lightweight materials, having goodelectrical conductivity and providing more efficient electrochemicalreactions, are desirable for use as electrode materials, as either thecatalyst support and/or the GDL.

In various energy storage devices, including capacitors, fuel cells, andbatteries, a current collector comprising a metal plate is typicallyattached to the exposed (outward-facing) surface of the electrode, tocollect the current generated by the device and conduct it towards themachine or equipment that the device is powering. Aluminum and copperare typical metals used as current collectors. It is desirable that theweight and complexity of the energy storage devices be reduced, and onesuch approach is to combine the function of the electrode with that ofthe current collector in a single material. This may only beaccomplished if both the conductivity and mechanical strength andintegrity of the material are near enough to those of traditionalcurrent collectors, such that the performance of the device is notdiminished. In fact, enhancement of the device performance by using acombined electrode/current collector would be ideal.

WO 2010/102250 discloses preparing cohesive carbon assembly bydispersing carbon in a liquid halogen (e.g. bromine), followed bysubstantial removal of the liquid. However, bromine is corrosive, highlytoxic, environmentally harmful, and expensive.

Therefore, there exists a need for a method for preparing a cohesivecarbon assembly that can avoid using the corrosive, toxic, and harmfulhalogen solvents, and provide a cost benefit.

SUMMARY OF THE INVENTION

The present invention is directed to a method of preparing a cohesivecarbon assembly comprising: (a) obtaining a functionalized carbonstarting material in a form of powder, particles, flakes, looseagglomerates, aqueous wet cake, or aqueous slurry; said functionalizedcarbon starting material is a carbon starting material which has beencovalently or non-covalently functionalized such that it is dispersiblein water; (b) dispersing the functionalized carbon starting material inan aqueous solution in a prescribed ratio to form a dispersion; and (c)substantially removing liquid from the dispersion in a controlledmanner, whereby the cohesive carbon assembly is formed.

The present invention is also directed to a cohesive carbon assemblyprepared by the method described above.

The present invention is further directed to applications of thecohesive carbon assembly in, e.g. electrical power storage andelectromagnetic interference shielding. The cohesive carbon assembly mayalso be used as an electrode and/or a current collector in a capacitor,fuel cell, or battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical image of nine aqueous CNT dispersions cast intoglass dishes, with the water evaporating under ambient conditions, asdescribed in Ex. 1.

FIG. 2 shows two optical images (A) and (B), of an aqueous CNTdispersion at different stages of water evaporation and formation of acohesive CNT assembly, as described in Ex. 1.

FIG. 3 shows two optical images (A) and (B), of nine aqueous CNTdispersions at different stages of water evaporation and formation ofcohesive CNT assemblies, as described in Ex. 1.

FIG. 4 shows optical images of (A) an intact but curled cohesive CNTassembly, and (B) the same assembly after being soaked with ethanol,pressed flat, and dried, as described in Ex. 1.

FIG. 5 shows optical images (A), (B), and (C) of cohesive CNT assembliesof various sizes, prepared as described in Ex. 2.

FIG. 6 shows optical images (A) and (B) of cohesive, adherent CNTassemblies deposited directly on aluminum foil substrates, prepared asdescribed in Ex. 3.

FIG. 7 shows optical images of a cohesive, adherent CNT assembly onpolyethylene terephthalate (PET) film, prepared as described in Ex. 4.;(A) CNT side, (B) PET side.

FIG. 8 shows a schematic diagram of an apparatus for continuous CNTwafer fabrication, as described in Ex. 5.

FIG. 9 is an electron microscope image of a cohesive, opticallytransparent CNT assembly over a TEM copper grid (with the copper gridvisible through the CNT assembly, prepared as described in Ex. 9.

FIG. 10 shows an optical image of an intact, cohesive 3-cm diameter CNTassembly prepared as described in Ex. 10.

FIG. 11 shows an optical image of a cohesive, adherent CNT assembly onan aluminum foil substrate, prepared as described in Ex. 11.

FIG. 12 shows (A) top view, and (B) side view optical images of anintact, cohesive 3-4 mm thick free-standing CNT assembly, prepared asdescribed in Ex. 12.

DETAILED DESCRIPTION OF THE INVENTION

One aspect of the invention relates to a method of preparing a cohesivecarbon assembly. The method comprises: (a) obtaining a functionalizedcarbon starting material in a form of powder, particles, flakes, looseagglomerates, aqueous wet cake, or aqueous slurry; said functionalizedcarbon starting material is a carbon starting material which has beencovalently or non-covalently functionalized such that it is dispersiblein water; (b) dispersing the functionalized carbon starting material inan aqueous solution in a prescribed ratio to form a dispersion; and (c)substantially removing liquid from the dispersion in a controlledmanner; whereby the cohesive carbon assembly is formed.

A cohesive assembly is defined herein as a self-assembled monolithicstructure in which the carbon is uniformly distributed; the cohesiveassembly has a distinct shape and size that is free-standing. Thecohesive assembly is further defined in that it has sufficientmechanical strength and integrity that it does not require mechanicalsupport by any other material, nor does it require the presence of abinder material to retain its strength, integrity, or functionalproperties. It also can be moved from place to place while retaining itsstructure, shape, and size.

The cohesive assembly is self-assembled in that, once the carbon in itsinitial form as described above is completely dispersed in a liquidmedium, no additional chemical modifications, physical alterations, ormechanical forces are applied to the carbon in order to form thecohesive assembly.

The carbon starting material may comprise carbon nanotubes (CNTs)selected from the group consisting of single-walled carbon nanotubes(SWCNTs), double-walled carbon nanotubes (DWCNTs), multi-walled carbonnanotubes (MWCNTs), and any combination thereof. The carbon startingmaterial may also comprise graphene, graphene oxide, graphite, expandedgraphite, exfoliated graphite, amorphous carbon, and any combinationsthereof. In one embodiment, the carbon starting material comprisesSWCNTs. In another embodiment, the carbon starting material comprisesDWCNTs. In a third embodiment, the carbon starting material comprisesMWCNTs.

In one embodiment, the carbon starting material comprising CNTs isobtained in a form of powder, particles, flakes, loose agglomerates,aqueous wet cake, or aqueous slurry, or any appropriate forms that canbe dispersed in water. In another embodiment, the carbon startingmaterial may be ground, pulverized, or mechanically altered in one ormore standard techniques to obtain the carbon starting material in anappropriate form before being dispersed in water.

A “functionalized carbon starting material”, as used herein, refers to acarbon starting material that has been physically or chemically modifiedto contain one or more functional groups linked to the carbon either bycovalent attachment or non-covalent attachment, such that the carbonstarting material is dispersible in water. “Dispersible” is definedherein as, capable of being dissolved or dispersed in water to form astable solution or suspension, at a concentration of at least 0.1 gramper gram of water. Typically, non-functionalized carbon startingmaterials are insoluble and non-dispersible in water (and many otherliquids).

Carbon starting materials including CNTs, graphene, graphene oxide, andgraphite may be functionalized by attaching one or more types ofchemical functional groups to the carbon starting material. In the caseof CNTs, this attachment may be to the walls and/or tube ends of theCNTs. This attachment may be achieved through physical or chemicalprocesses. CNT starting materials may also be functionalized bydepositing a substance or substances inside the tubes of the CNTs.

A preferable method of producing functionalized CNTs is to attachappropriate chemical functionalities onto the conjugated sp² carbonscaffold of the CNTs. One broad category of such functionalitiesincludes polar groups that associate with water molecules throughhydrogen bonding. Such derivatization of CNTs with polar moieties may beperformed through covalent functionalization or non-covalent approaches.

Examples of covalent functionalization are attachment of acyl chloride(—COCl), hydroxyl (—OH), carboxyl (—COOH), or organic ester (—COOR)groups. Acyl chloride, hydroxyl, and carboxyl groups can be either thefinal terminal groups themselves, or may be used as precursor(intermediate) groups for subsequent attachment of other functionalgroups. For example, carboxylic acid-terminated CNT (—COOH) serves asthe precursor for covalent attaching of a host of water-solubilizingpolymers, dendrimers, oligomers, and proteins. Examples offunctionalized CNT that proceed through the —COOH terminal group includepolyethylene oxide (PEO)-functionalized CNT, polyethylene glycol(PEG)-functionalized CNT, polyvinyl acetate (PVA)-functionalized CNT,amino terminated polystyrene-functionalized CNT, and bovine serumalbumin (BSA)-functionalized CNT.

Functional groups include, but are not limited to, carboxy, amide,hydroxy, glycol, ether, hemiacetal, hemiketal, amino, methacrylate,thiol, carbonyl, urethane, pyrrole, aniline, cyano, aminoacyl,acylamino, alkyl ester, and aryl ester. Carboxy, amide, hydroxyl, andglycol are preferred.

Examples of non-covalent functionalization are association with pyrenederivatives containing hydrophilic groups. The interaction in this casedoes not involve formation of covalent chemical bonds between functionalgroups on the CNT and the pyrene moiety. Instead, the non-covalentfunctionalization of CNT proceeds through Van der Waals π-π electronicinteractions between the CNT side walls and the polyaromatic pyrenemolecules. The presence of hydrophilic groups such as ammonium ions onthe pyrene moieties then allow for CNT water dispersibility.

The degree of CNT functionalization is typically between about 0.01% andabout 30% (as mole percentage of functional groups), preferably betweenabout 0.05% and about 20%, more preferably between about 0.1% and about10%, and yet more preferably between about 1.0% and 6.0%.

Functionalization of CNTs as described above may occur through methodsor processes that are specifically intended to functionalize them, or asa side effect of processes applied to the CNTs for other purposes. Forexample, in industrial CNT manufacturing, treatment with strong acidssuch as HCl, HNO₃, and/or H₂SO₄ is often used to remove metallic and/oroxide impurities such as residual catalyst support and catalyst. Thisacid treatment may result in, for example, carboxylated oracid-sulfonated functionalized CNTs with high dispersibility in water.This may occur in the purification of SWCNT, DWCNT, or MWCNT, or anycombination thereof. Use of intentionally or unintentionallyfunctionalized CNTs, or both, as SWCNT, DWCNT, MWCNT, or any combinationthereof, is within the scope of the present invention.

Functionalized CNTs are also commercially available. Examples includesingle-walled carbon nanotubes sold by Carbon Solutions Inc., Riverside,Calif. under the product grades P3 (1-3 atomic percent carboxyfunctionalized), P7 (PEG-derivatized, between approximately 5 and 20 wt%), P8 (m-polyaminobenzene sulfonic acid (PABS)-derivatized, betweenapproximately 50 and 65 wt %), and P9 (between approximately 0.5 and 5.0atomic percent amide group functionalized).

Baytubes® C 150 P multi-walled CNT, available from Bayer MaterialScience(Leverkusen, Germany), is an example of a commercial MWCNT product thatis functionalized as a side effect of a process applied for anotherpurpose. Through acid purification, likely performed using nitric acid,sulfuric acid, or a combination thereof, the MWCNT may be terminatedwith carboxylic acid (—COOH), hydroxy (—OH), or both functional groups,making the material dispersible in an aqueous medium. The degree offunctionalization is estimated to be about 0.1-0.5 atomic percent.

The functionalized CNTs may supplied in the form of wetcake (looseagglomerates in a liquid), as a slurry, as a dispersion in water, or asdry particles. The wetcake material may be dried by any standard method,then mechanically broken apart into particles or loose agglomerates, andthen used in the preparation of the cohesive carbon assemblies.Optionally the dried wetcake material may be further ground into smallerparticles or powder, and then used in the preparation of the cohesivecarbon assemblies. The wetcake, slurry, or dispersion may be used inpreparation of the assemblies in the as-received condition, or it may bedried and optionally mechanically broken apart before use. The dryparticles, which are typically smaller than 5 mm in the largestdimension, may be used as-received in the preparation of the cohesivecarbon assemblies. Optionally, this material may be ground into smallerparticles or powder and then used in the preparation of the cohesivecarbon assemblies. Generally speaking, the powder, particles, flakes, orloose agglomerates of carbon used in the invented method are smallerthan 1 cm in the largest dimension, preferably smaller than 3 mm in thelargest dimension, and more preferably smaller than 1 mm in the largestdimension.

The aqueous solution used in the method of the invention may compriseany kind of water, including tap water, bottled water, distilled water,de-ionized water, D₂O, or any combination of these. In one embodiment,the aqueous solution is water, preferably it is at least 90% pure, or95% pure, or 99% pure. In another embodiment, the aqueous solutioncontains dissolved ions or salts at a concentration of up to 20 weightpercent in water. The aqueous solution may be acidic, alkaline, orneutral, and may have a pH anywhere within the range of about 1.5 to14.0. A buffer may be added to the aqueous solution at a concentrationof up to 10 weight percent.

In one embodiment, the aqueous solution does not contain any organicsolvent. In another embodiment, a small amount of organic solvent, suchas ethanol, 2-propanol, acetone, and the like, may be present in theaqueous solution. Up to about 5 weight percent of organic solvent may beadded. If the amount of organic solvent is higher than this, theadvantage of being environmentally friendly is lost, because uponevaporation a large amount of organic solvent may be released to theatmosphere, unless proper equipment is used to recover the liquid. Also,any waste solution generated in the process would need to be handled fordisposal as an organic liquid, a hazardous material. Furthermore, thevolatility of the dispersing solution may rise as the quantity oforganic solvent is increased, making the process of evaporating thedispersing liquid unpredictable and difficult to control.

In step (b), the carbon starting material is dispersed in the aqueoussolution, in a prescribed ratio. A prescribed ratio of a carbon startingmaterial to the aqueous solution is defined as a ratio that will resultin dispersion of the carbon in the aqueous solution, and in theformation of the cohesive assembly when liquid is removed. For aparticular type of carbon starting material, there is a range ofprescribed ratios that are determined experimentally. Within that rangeof prescribed ratios, that type of carbon starting material willdisperse in the aqueous solution and can form a cohesive assembly whenliquid is removed in a controlled manner.

If the ratios of the carbon starting material and aqueous solution (or“solution”) amounts are outside the range of the prescribed ratios forthat particular type of carbon starting material, a cohesive carbonassembly will not form. For example, if the ratio of the carbon startingmaterial to solution is too high, the carbon starting material may notdisperse completely in the solution, but rather remain as powder,particles, flakes, or loose agglomerates, which may appear floating orsuspended in the solution, or settle to the bottom of the solution inthe container. If the ratio of carbon starting material to solution istoo low, the carbon starting material may disperse completely. It maythen form into an assembly during removal of liquid, but then break intopieces at the end of the process. Or, the dispersed carbon may assembleinto particles or flakes, but not into a monolithic cohesive assembly.Or, the dispersed carbon may simply remain as a residue of powder,particles, flakes, or loose agglomerates in the container when liquid isremoved.

In one embodiment, the prescribed ratio of the carbon starting materialto solution may be between about 0.015 and about 200 mg per gram ofsolution, between about 0.01 and about 50 mg per gram of solution,between about 0.05 and about 50 mg per gram of solution, between about0.1 and about 30 mg per gram of solution, or between about 0.3 and about10 mg per gram of solution. “About”, as used herein, refers to +/−10% ofthe recited value.

In one embodiment, the carbon starting material comprisingfunctionalized SWCNTs is dispersed in the solution in a prescribed ratioof between about 0.1 and about 20 mg carbon starting material per gramof solution.

Dispersing, as used herein, is forming a stable suspension of carbon inthe solution. A stable suspension is one in which no visible powder,particles, flakes, or loose agglomerates precipitate out of the solutionor settle to the bottom of the mixture when no mechanical agitation isapplied. In one embodiment, to disperse the carbon in the solution, thecarbon is first combined with the solution in a container to form amixture, and then the mixture is mechanically agitated by one or morestandard methods, for example, without limitation, mechanical stirring,and/or sonication (bath sonication, probe sonication, or a combinationof the two), and/or microfluidization. In another embodiment, the carbonis first combined with the solution in a container to form a mixture,then the mixture is simultaneously mechanically agitated and refluxed ator near the solution boiling temperature for between 0.1 and 20 hours,preferably between 1 and 6 hours, and then the mixture is mechanicallyagitated by sonication and/or microfluidization.

Dispersion of the carbon starting material in the solution in step (b)may be carried out at a suitable temperature under a suitable pressurewherein the solution is in a liquid form, i.e. above the melting pointand up to and including the boiling point of the solution under asuitable pressure. In one embodiment, the carbon starting material isdispersed in the solution at atmospheric pressure, at a temperatureabove about 0° C. and up to about 100° C., between ambient roomtemperature and about 45° C., or between 10° C. and 30° C. Ambient roomtemperature (about 20° C.) and pressure are typically suitableconditions. In another embodiment, the carbon starting material isdispersed in the solution at about 100° C. by refluxing.

Dispersion of the carbon starting material in the solution may becarried out in the presence of one or more types of mechanicalagitation. The dispersion step may comprise more than one periods ofmechanical agitation. In each period, one or more types of mechanicalagitation may be carried out. The same type of mechanical agitationcarried out at different periods may have the same or differentparameters. In one embodiment, the dispersion of carbon startingmaterial in the solution comprises two periods of mechanical agitation,the first period comprising sonication and the second period comprisingmicrofluidization. In another embodiment, the dispersion of carbon inthe solution comprises two periods of mechanical agitation, the firstperiod comprising mechanical stiffing and the second period comprisingmicrofluidization.

Mechanical agitation may be carried out using standard laboratorymagnetic stirring plate and magnetic stir bars immersed in the mixtureof carbon and solution. Alternatively, mechanical agitation may becarried out using a high shear mixer comprising a rotor or impeller,together with a stationary component known as a stator, or an array ofrotors and stators. The mixer is used in a tank containing the carbonstarting material and the solution mixture to be mixed or in a pipethrough which the mixture passes, to create shear.

Microfluidization may be carried out using commercially availableequipment, for example, that produced by Microfluidics Corp., Newton,Mass.

Sonication may be carried out by a variety of methods using commerciallyavailable equipment, examples include, without limitation, an ultrasonicprocessor with a probe or wand, and an ultrasonic bath or tank.Sonication may be carried out for a suitable time period at a suitableenergy level at a suitable temperature. In one embodiment, the suitabletime period is between about 1 minute and about 100 hours, between about5 minutes and about 5 hours, or about 20 minutes. The suitable energylevel is at least 0.01 watt/gram of solvent, or between 0.16 watt/gramof solvent and about 1.6 watt/gram of solvent. The suitable temperatureis the same as described supra.

The dispersion of carbon starting material in the solution in step (b)is different from commonly known methods of carbon dispersion, and inparticular, CNT dispersion, in that no surfactant chemicals are neededto disperse the carbon starting material. In one embodiment, the carbonstarting material is dispersed in the solution substantially free ofsurfactants. Surfactants are typically used to disperse carbon, and morespecifically, carbon nanotubes, in a liquid, and in known methods ofpreparing carbon assemblies, surfactants are usually present as aresidue. Examples of such surfactants include but are not limited tocetyl trimethylammonium bromide (CTAB), dodecylbenzenesulfonic acidsodium salt (NaDDBS), sodium cholate, sodium dodecyl sulphate (SDS),polyoxyethylene (10) octylphenol (Triton X-100) and poly(ethylene oxide)(20) sorbitan mono-oleate (Tween 80). “Substantially free ofsurfactants,” as used herein, means less than 10%, preferably less than1%, and more preferably less than 0.1% (w/w) of surfactants is presentrelative to the weight of carbon starting material used to prepare theassembly. Such surfactants are not needed to disperse the carbon in thesolution, when the carbon is dispersed in a solution according to themethod of the invention.

Typically, ionic surfactants such as cetyl trimethylammonium bromide(CTAB), dodecylbenzenesulfonic acid sodium salt (NaDDBS), sodiumcholate, and sodium dodecyl sulphate (SDS), or nonionic surfactants suchas polyoxyethylene (10) octylphenol (Triton X-100, Dow Chemical Co.) andpoly(ethylene oxide) (20) sorbitan mono-oleate (Tween 80, ICI Americas,Inc.) are needed to effectively disperse CNTs in a liquid medium. Thesesurfactants, when used to disperse CNTs, may remain as a residue andthereby degrade the electrical or mechanical properties of the finalCNT-derived product. The cohesive assembly, when prepared by the presentmethod, need not contain surfactants. Therefore, the method of thecurrent invention represents a substantial improvement over existingtechniques for dispersing CNTs in an aqueous medium.

Furthermore, the carbon starting material is dispersed in the solutionthat is substantially free of a binding material (e.g. polymers,inorganic or hybrid materials). For industrial use, such bindingmaterials are typically required in order to form a carbon monolith. Forexample, to form a monolith of activated carbon for use in anelectrochemical double layer capacitor (EDLC), a polymer bindingmaterial such as PTFE (polytetrafluoroethylene) is needed to hold thecarbon particles together. Similarly, to form carbon aerogel monolithstypically requires impregnation with an organic-based aerogel that actsas a binder and is then later removed by pyrolysis. In the method of thecurrent invention, no such material is needed in order to form thecohesive carbon assembly as a monolith. “Substantially free of a bindingmaterial,” as used herein, means less than 10%, preferably less than 1%,and more preferably less than 0.1% (w/w) of binding material is presentrelative to the weight of carbon starting material used to prepare theassembly.

In certain embodiments, the carbon-solution dispersion may be applied toa surface after step (b). In one embodiment, the surface comprises ahydrophobic surface (e.g. a surface comprising a dimethyl organosilane,a fluorinated dimethyl organosilane, a fluorinated polymer, Teflon, or acombination thereof). In another embodiment, the surface comprises ahydrophilic surface (e.g. metal (e.g. aluminum, copper, gold, silver,platinum, tantalum, titanium, stainless steel, and other electrodematerial) surface, glass surface, silicon surface, plastic, andceramic.) To achieve a self-delaminating CNT wafer, the method furthercomprises applying the carbon-solution dispersion to a hydrophobicsurface having water contact angle of at least about 80°. To achieve acoherent adhesive CNT coating, the method further comprises applying thecarbon-solution dispersion to a hydrophilic surface having water contactangle of less than about 80°.

The dispersion may be applied to the surface by any known method, e.g.without limitation, spin-coating, dip-coating, flow-coating, spraycoating, casting, or any combination thereof. The spin-coating may becarried out at a spinning speed of about 10 rpm to about 10,000 rpm, orabout 300 rpm to about 5,000 rpm for a time of at least about 5 seconds.The dip-coating may be carried out at a withdrawing speed of about 0.01to about 1.0 cm/s, about 0.1 to about 0.4 cm/s, or about 0.2 cm/s.

In other embodiments, a carbon-solution dispersion may be transferredfrom one container to another container after step (b). One objective ofsuch transferring is to produce a cohesive carbon assembly of certaindesired size and shape. The amount of dispersion transferred, and thesize and shape of the container, can be selected to produce assembliesof various sizes, shapes, and thicknesses. Another objective oftransferring the dispersion is to produce a large number of individualassemblies of desired size(s) and shape(s) from a single large batch ofcarbon-water dispersion, thereby increasing efficiency and improvingcost performance of the process. The container into which the dispersionis transferred may have surfaces comprising a hydrophobic surface,examples of which are given above. Or, the container into which thedispersion is transferred may have surfaces comprising a hydrophilicsurface, as also described above.

The cohesive carbon assembly will form and remain intact (crack- anddefect-free) when the ratio of the amount of CNT in the dispersion tothe surface area onto which it is applied, is within a certain range.The suitable ratio of amount of CNT to the applied surface area(milligrams per cm²) is between about 0.01 mg and about 500 mg per cm²,and preferably between about 0.1 mg and about 50 mg per cm². In oneembodiment, the CNT-solution dispersion is applied onto a surface insidea container such that the ratio of CNT to bottom surface area of thecontainer is between about 1 and about 5 mg per cm².

In step (c), the liquid portion of the dispersion is substantiallyremoved, i.e. greater than 99% of the liquid is removed, in a controlledmanner, whereby the cohesive assembly of carbon is formed. In order forthe cohesive assembly to form, liquid must be removed in a controlledmanner. “Removing in a controlled manner,” as used herein, refers toremoving liquid under conditions such that the dispersed carbonself-assembles into the cohesive assembly of carbon, and the assemblyremains intact as a single cohesive monolith throughout the removalprocess, and after liquid removal is completed. Any method to removeliquid in a controlled manner that allows the self-assembly of thecarbon into a cohesive assembly, and allows the assembly to remain as acohesive monolith after liquid removal is completed, is within the scopeof the invention. For example, without limitation, a controlled mannerof removing liquid may include evaporation, draining of liquid from thecontainer, or a combination thereof. It is important not to removeliquid so rapidly such that it will disturb or prevent the carbon fromforming a cohesive monolith. It is also important not to agitate themixture during the removal process.

An example of a non-controlled manner of removing liquid is pouring offthe water by tipping the container (decanting), as this would clearlydisturb the formation of the cohesive assembly and not result in amonolithic form. Another example of a non-controlled manner is boilingof liquid, as the accompanying vapor bubble generation and resultantagitation of the mixture would clearly disturb the cohesive assembly andprevent the monolith from forming. A third example of a non-controlledmanner would be direct physical removal of the liquid at or through itsexposed top surface in the container, for example, by suctioning througha tube or pipe. The breaking of the surface of the liquid by the tube orpipe would clearly interfere with the self-assembly of the carbon into amonolith.

In one embodiment, the controlled removal of liquid is conducted byevaporation. During the initial stages of this evaporation, thedispersed carbon first nucleates on the top surface of the liquid, andthen begins to assemble or coalesce into “islands” of carbon on thesurface of the liquid. As evaporation progresses, the islands grow andjoin together to form larger islands, eventually joining into a singlemonolithic disc, wafer, or film, i.e., a cohesive assembly of carbon.

If liquid is evaporated too quickly, a cohesive assembly of carbon mightnot form. In such instances, the carbon may not nucleate on the topsurface of the liquid, but may instead remain as a powder or particleresidue in the container. Or, the carbon may nucleate on the surface,and islands may begin to form, but they will not coalesce into amonolithic cohesive assembly, and remain as randomly-shaped agglomeratesof carbon rather than a cohesive assembly. Or, the islands may coalesceinto a monolith, but then later break apart into smaller pieces.

The specific conditions for controlled removal of liquid, that willresult in the formation of a cohesive assembly of carbon, depend on thetype of the carbon starting material and the ratio of the carbon to thesolution, and can be determined experimentally. For example, liquid isremoved by evaporation at a suitable pressure, at a suitabletemperature, and for a suitable time.

The suitable pressure may be between about 0.001 Torr and about 5,000Torr, between about 0.01 Torr and about 1500 Torr, between about 1 Torrand about 1000 Torr, between about 100 Torr and about 800 Torr, or atatmospheric pressure (about 760 Torr).

The suitable temperature for controlled removal of liquid may be betweenabout 0° C. and about 100° C., between about 10° C. and about 60° C.,between about 20° C. and about 40° C., or at ambient room temperature(between about 15° C. and about 30° C.). In one embodiment, thecontrolled removal of liquid is achieved at an ambient room temperaturebetween 18° C. and 25° C.

The suitable time for controlled removal of liquid may be between about1 minute and about 300 hours, between about 10 minutes and about 200hours, between about 30 minutes and about 100 hours, or between about 1hour and about 50 hours.

In one embodiment liquid is removed by evaporation at atmosphericpressure. In another embodiment, liquid is removed in a closed system ata pressure below atmospheric pressure. Either condition may beaccompanied by heating to accelerate the evaporation of liquid, providedthat the rate of evaporation is controlled such that formation of thecohesive assembly of carbon is not disturbed or prevented.

In one preferred embodiment, liquid is removed by evaporation atatmospheric pressure and ambient room temperature (ambient conditions),by simply exposing the carbon-solution dispersion to the ambientenvironment. This may be achieved whether the dispersion is applied to asurface or enclosed in a container. In this embodiment, the evaporationis controlled only by the ambient conditions.

The evaporation of liquid may alternatively be controlled to form acohesive assembly, by controlling the ambient relative humidity aroundthe carbon-solution dispersion. This may be achieved by, for example,enclosing the dispersion in a controlled humidity chamber. This approachis especially suitable when the dispersion is applied to a surface as acoating or film. Humidity control may also be achieved by, for example,placing a partial barrier enclosing the container in which thedispersion is held.

The evaporation of liquid may alternatively be controlled to form acohesive assembly, by monitoring the evaporation rate of liquid andmaintaining it within a range that will not prevent or disturb theformation of the assembly. The lower end of the operable range ofevaporation rates is not particularly limited, except that a very lowrate will result in an impractically long time to produce the cohesiveassembly. The evaporation of liquid typically follows the classic andwell-known theory of two-stage drying of porous bodies first proposed byThomas K. Sherwood in “The Drying of Solids—I”, Industrial Engineeringand Chemistry 21, 1 (1929), 12-16, and in “The Drying of Solids—II”,Industrial Engineering and Chemistry 21, 10 (1929), 976-980. During thefirst drying stage, also known as the Constant Rate Period, theevaporation rate is preferably between about 0.01 and about 50milliliters/minute (ml/min), more preferably between about 0.10 andabout 5.0 ml/min. During the second drying stage, also known as theFalling Rate Period, the evaporation rate is preferably between about5×10⁻⁵ ml/min and about 5×10⁻² ml/min, more preferably between about5×10⁻⁴ and about 7×10⁻³ ml/min.

Typically, greater than 99% of liquid is removed by evaporation. Anyremaining liquid may optionally be removed after evaporation, by rinsingthe cohesive assembly with water or an organic solvent such as ethanolor isopropanol, and then drying the assembly either at room temperatureor with mild heating in an oven.

A cohesive carbon assembly formed inside a container may be removed fromthe container manually or by lightly rinsing the inner surfaces of thevessel with a fluid such as a dilute acid, base, aqueous solution, ororganic solvent. The product assembly may then receive a final drying atatmospheric pressure or under vacuum, which may be accompanied by mildheating. Typically, cohesive assemblies formed on a hydrophobic surfacewill be easily removed from the surface or container, and will requirelittle or no rinsing. Cohesive assemblies formed on hydrophilic surfacewill typically require at least some rinsing or mechanical force toremove them from the surface or container.

Characterization of Cohesive Carbon Assemblies

Cohesive carbon assemblies prepared by the method of the invention arecharacterized by the substantial absence of surfactants during thepreparation and in the final product.

Cohesive carbon assemblies prepared by the method of the invention arealso characterized by the absence of halogen residue. Assembliesprepared by previously disclosed methods employing halogens as thedispersing solvent typically retain a large amount of halogen as aresidue in the assembly. Such residue may be present as solid freehalogen deposited on the assembly, or as halogen molecules bonded to thecarbon structure. In either form, the halogen residue can degrade theperformance and usefulness of the assembly, by interfering with itselectronic properties. In particular, such residue may cause increasedcontact resistance when the assembly is placed in contact with anothercomponent, such as a current collector in a capacitor, fuel cell, orbattery. Furthermore, the amount of residue that may be present on theassembly is highly unpredictable, so the use of halogens may result ininconsistent properties and performance among different assemblies. Itis thus highly advantageous to have a cohesive carbon assembly preparedusing a dispersing medium that does not leave such residue. Use of anaqueous solution, and pure water in particular as the dispersing medium,reduces or eliminates this residue.

Cohesive carbon assemblies comprising CNTs, prepared by the method ofthe invention, feature high effective carbon packing density compared toother known CNT assemblies. The cohesive carbon assemblies typicallyhave effective CNT packing density of at least about 0.5 g/cm³, oftenhave densities higher than 1.0 g/cm³, and have shown densities as highas 1.5 g/cm³. For example, the cohesive carbon assemblies have effectiveCNT packing density of between about 0.3-1.9 g/cm³, preferably betweenabout 0.5-1.5 g/cm³, and more preferably between about 0.8-1.5 g/cm³ orbetween 1.0-1.5 g/cm³. This high density imparts these assemblies withgood mechanical strength and integrity. This high density alsocontributes to their superior electrical properties; in particular theirlow resistivity compared to other known CNT assemblies.

To determine the effective CNT packing density in a CNT-derived carbonassembly, first the apparent density of the assemblies is determined bycarefully measuring the weight of the assembly using a standardanalytical balance, then measuring the dimensions of the assembly usinga digital micrometer or optical or scanning electron microscope, thencalculating the volume of the sample from the dimensions, and dividingthe weight by the volume. This calculation provides the apparent densityof the assembly. Alternatively, the apparent density may be determinedusing a density balance and Archimedes' principle. Then, using one ofvarious methods such as energy dispersive x-ray spectroscopy (EDS),neutron activation analysis (NAA), or thermogravimetric analysis (TGA),the weight fraction of carbon (i.e., CNTs) in the assembly can bedetermined. Finally, the effective packing density of CNTs is calculatedby multiplying the apparent density by the weight fraction of carbon inthe assembly.

The assemblies can be produced in a desired size or shape, which isdetermined by the amount of carbon used to prepare the assembly, and bythe size and shape of the container in which the carbon assembly isprepared. This may allow the assemblies to be used in variousapplications requiring carbon assemblies of various shapes and sizes.When liquid is removed from the dispersion, the carbon typicallyself-assembles in the shape and size of the bottom of the vessel in thehorizontal plane, with a vertical, i.e., perpendicular thickness that isdetermined by the amount of carbon used and the size of the container.Greater amounts of carbon will produce a thicker wafer or disc-likecohesive assembly, while less carbon will produce a thinner, film-likeassembly. Decreasing or increasing the diameter or cross-sectional areaof the container used to prepare the assembly has similar effects onassembly thickness. In certain embodiments, the assembly has a thicknessof about 0.02 μm to about 2,000 μm, or about 0.1 μm to about 500 μm. Inone embodiment, the assembly is a self-delaminating assembly having athickness of about 0.1 μm to about 2000 μm, about 1 μm to about 500 μm,or about 10 μm to about 50 μm. In another embodiment, the assembly is anadhesive assembly having a thickness of about 0.02 μm to about 2000 μm,about 0.02 μm to about 500 μm, or about 0.02 μm to about 50 μm.

Cohesive carbon assemblies may be prepared by the method of the presentinvention with sufficiently small thickness, such that they showsignificant optical transmittance. Above a certain thickness, opticaltransmittance is negligible, and as thickness is decreased, thetransmittance increases, to a maximum of greater than 90% in thewavelength range of 500-2000 nm. The transmittance of a particularcohesive assembly depends on factors such as the type of carbon(s)present in the assembly, the packing density of the assembly, andprocessing parameters such as drying conditions, substrate type, etc.Generally speaking, a cohesive carbon assembly prepared by the presentmethod having thickness between about 0.02 μm and about 1.0 μm may haveoptical transmittance of up to about 97%. In one embodiment, a cohesivecarbon assembly is prepared by the method of the invention, havingthickness between about 0.10 and 0.15 μm, and optical transmittance ofbetween about 80 and 90% between 500 and 2000 nm.

The cohesive carbon assemblies prepared by the method of the inventionalso feature low electrical resistivity compared to other carbonassemblies. These assemblies typically have resistivity of less thanabout 0.1 Ω-cm, about 0.02-0.05 Ω-cm, and an electrical sheet resistanceof less than about 2,000Ω per square, or between about 8 and about 17Ωper square. This low electrical resistivity along with mechanicalstrength and integrity may allow various applications of theseassemblies, for example, as electrodes for batteries or supercapacitors,or as electromagnetic interference (EMI) shielding materials. This lowresistivity is related to the high effective carbon packing density ofthe assemblies in that as this density increases, empty space betweenindividual carbon entities such as nanotubes, tube bundles, or graphiteplatelets decreases, and the area of contact between these carbonentities increases. This naturally leads to more efficient and highercurrent flow through the assembly, thereby decreasing its resistivity.

Resistivity of the cohesive assembly is determined as follows: From eachassembly, a sample of rectangular or square geometry is cut thatpossesses lengths greater than 1 cm on all sides. The sample is mountedin a sample mount, and two electrical contact pairs (two currentcarrying and two voltage sensing) are directly compressed to the sample,in a standard Kelvin-type (4-point) probe configuration. The sample ispositioned such that the four metal tips of the four-point probe makedirect contact with the sample without puncturing through it.

A constant current is made to flow the length of the sample by using ahigh impedance current source. The current source is typically set toapply a current of 0.1×10⁻³ A, 1×10⁻³ A, 10×10⁻³ A, or 100 ×10⁻³ A. Thevoltage drop across the sample is measured using a high impedancedigital voltmeter. The surface (sheet) resistance, R_(s) in Ω (or Ω/sq),of the sample is the ratio of the stable voltage registering on thevoltmeter, V, to the value of the output current of the current source,I, multiplied by the geometric factor π/ln2≈4.53:

R _(s)=4.53 (V/I).

By measuring the thickness (t) of the sample, using a profilometer,digital micrometer, or scanning electron microscope, the electricalresistivity ρ of the sample in Ω-cm, can be calculated using theformula:

ρ=R _(s)(t)

Additionally, cohesive carbon assemblies prepared by the method of theinvention using carbon starting material comprising CNTs (e.g. SWCNTs)display no more defects than the carbon starting material. A knowntechnique useful for evaluating the quality of CNTs, i.e., theconcentration of structural defects and amorphous carbon impuritiesincluded therein, is by measuring the intensity ratio of twocharacteristic Raman infrared spectral peaks, called the G/D ratio. TheG-line is a characteristic feature of the graphitic layers andcorresponds to the tangential vibration of the carbon atoms. The D-lineis a typical sign for defective graphitic structures. When determiningthe quality level of a CNT sample via Raman spectroscopy, the absoluteintensities of the G and D band peaks are not particularly relevant.Rather, the ratio of the intensity of the two peaks is the relevantmeasure. The comparison of the ratios of these two peaks' intensitiesgives a measure of the quality of the CNT samples. Generally, the G/Dratio is the ratio of good to bad CNT peaks. Thus, CNTs having a higherG/D indicate a lower amount of defects and a higher level of quality.

A G/D ratio is typically determined using a Raman spectroscopytechnique. Any of various commercially available instruments may be usedto measure the G and D band intensities and to calculate the G/D ratio.One example of such an equipment is available from HORIBA Jobin YvonInc., Edison, N.J., under the model name LabRAM ARAMIS.

In a CNT sample, the G/D ratio may change after treatment. The presentmethod has the advantage that the G/D ratio of the formed cohesivecarbon assembly is about the same or greater than the G/D ratio of thecarbon starting material, indicating that the method does not introducestructural defects during the process.

Applications of Cohesive Carbon Assemblies

Another aspect of the invention relates to an article comprising asubstrate and a cohesive carbon assembly coated onto at least onesurface of the substrate, wherein the cohesive carbon assembly has beenprepared by the method described supra.

The cohesive carbon assembly may be otherwise treated after itsfabrication, in order to enhance its performance for certainapplications. For example, for application as a fuel cell electrode, acoating of metal particles, such as platinum, may be advantageous forits catalytic properties. For battery electrode applications, metalparticle coatings such as iron, platinum, palladium, nickel, lithium, orother appropriate metals may be desired. Such particle coatings may beaccomplished using a method disclosed by Grigorian et al in US PatentApplication Publication US 2009/0015984A1, which is hereby incorporatedby reference.

The cohesive carbon assembly of the present invention has particularadvantages over other types of carbon assemblies for use as an electrodeor current collector in electrochemical capacitors, fuel cells, orbatteries. These advantages include its inherent mechanical strength andintegrity, low electrical resistivity, ability to be fabricated and/orfurther modified to a desired shape and size, and high carbon packingdensity that results in excellent energy storage capabilities (i.e.,power density and energy density).

The cohesive carbon assembly is appropriate for use as an electrode in acapacitor or a capacitor cell, which are used interchangeably in thisapplication, due to its desirable combination of electrical andmechanical properties. The capacitor may be of any type that comprisestwo electrodes separated by an insulating material. The capacitor may bea simple electrostatic capacitor with a bulk dielectric materialseparating the two conducting electrodes, or an electrolytic capacitor,in which one or both of the electrodes comprise(s) an electrolyte. Thecohesive carbon assembly is especially suitable for use as an electrodein an electrochemical double-layer capacitor (EDLC), sometimes referredto as a “supercapacitor” or “ultracapacitor”.

The cohesive carbon assembly, and in particular the assembly comprisingcarbon nanotubes, may be altered after fabrication by the inventedmethod into an electrode of suitable size or shape for directinstallation into a capacitor cell. The electrode may be disc-shaped,i.e. round or ovoid, or it may be a polygon having three or more sides.The size and shape are determined only by the size and shape of thecapacitor device in which it will be used. The thickness of theelectrode is not particularly limited, but certain thicknesses may bepreferable for use in capacitor devices. If the electrode is too thick,resistance of the electrode may be too high or energy transfer will beinefficient. If it is too thin, it will not have the necessarymechanical integrity or energy storage potential for capacitor use.Generally, the thickness is preferably between about 0.02 μm and about2,000 μm, or between about 0.1 μm and about 500 μm. In one embodiment,the assembly is a self-delaminating assembly having a thickness of about0.1 μm to about 2000 μm, about 1 μm to about 500 μm, or about 10 μm toabout 50 μm. In another embodiment, the assembly is an adhesive assemblyhaving a thickness of about 0.02 μm to about 2000 μm, about 0.02 μm toabout 500 μm, or about 0.02 μto about 50 μm.

The cohesive carbon assembly may be optionally purified of metallicimpurities prior to use as a capacitor electrode. Specifically, for anassembly comprising carbon nanotubes, removal of metallic impuritiesthat are residues of the CNT synthesis process may improve theelectrical and energy storage properties of the assembly. Thispurification may be accomplished by various means, with treatment with ahalogen gas, and chlorine gas in particular, being the preferablemethod. The parameters of this treatment process are not particularlylimited, provided the carbon is not damaged or degraded during theprocess.

To evaluate the performance of a cohesive carbon assembly as a capacitorelectrode, one electrode may comprise a cohesive carbon assembly in anasymmetrical capacitor cell, or two electrodes may each comprise acohesive carbon assembly in a symmetric capacitor cell. The method ofevaluating the performance of the cohesive carbon assembly as acapacitor electrode is not particularly limited, and there are variousstandard methods known in the field. Typically, the capacitor cellcomprising the two electrodes separated by an insulating material isassembled with metal plates as current collectors attached to the outersurfaces of the electrodes. The cell is then submerged in an appropriateelectrolyte and a voltage is applied. For EDLCs, the preferable appliedvoltage (absolute value) is between 0 and 2 volts, or between 0 and 4volts, to evaluate performance for consumer electronics and vehicleapplications. Analytical methods used to evaluate the electrodeperformance may include leakage current measurement, electrochemicalimpedance spectroscopy (also known as dielectric spectroscopy),charge/discharge cycling using commercially available test equipment,and the like.

To determine the performance advantage of the cohesive carbon assemblyas a capacitor electrode, the properties measured are compared to thoseof capacitors comprising electrodes of other standard materials such asactivated carbon, or other types of CNT-based electrodes such as CNTforest-derived materials. Cohesive assemblies of carbon prepared by thepresent method in general show superior power performance as capacitorelectrodes, compared to activated carbon electrodes and other types ofCNT-based electrodes. The superior performance includes lower leakagecurrent and faster discharge time, and a better combination of powerdensity and energy density, important parameters for electric vehicleand consumer electronics applications. Furthermore, the cohesiveassemblies possess the necessary mechanical integrity to be packageddirectly into sealed capacitor cells, whereas the other CNT-basedelectrodes do not.

Similarly as for a capacitor, the cohesive carbon assembly of thepresent invention is suitable for use as an electrode in a battery. Thebattery may be of any type comprising two electrodes separated byelectrolyte. Of particular interest is the Li-ion battery type, in whichthe cohesive carbon assembly is suitable for use as the anode or cathodematerial, or both. As for the capacitor application, the size, shape,and thickness of the battery electrode comprising the cohesive carbonassembly are not particularly limited. Preferred thicknesses are alsosimilar to those for capacitor electrodes.

The cohesive carbon assembly may be used as a battery electrode in itsas-prepared form, i.e. as an assembly comprising nearly pure carbon. Or,the assembly may be further treated after it is fabricated by, forexample, coating with metal particles using the method described in USPatent Application Publication US 2009/0015984A1. The metal coating maybe selected such that the assembly is suitable for use as the anode, orit may be selected such that the assembly is suitable for use as thecathode. The appropriate metal coating depends on the overall design ofthe cell.

In its as-prepared form, a cohesive carbon assembly of carbon nanotubes,and more preferably, a cohesive assembly of SWCNT, is especiallyappropriate for use as the anode in a Li-ion battery cell, with acorresponding cathode comprising one or more Li-containing oxides suchas LiCoO₂, LiFePO₄, or LiNiCoAlO₂. The electrode comprising the cohesivecarbon assembly requires no binder material and can be installed in abattery cell in its as-prepared form.

A battery containing a cohesive carbon assembly electrode may beperformance tested using a standard method such as is described by Y.NuLi et al in “Synthesis and characterization of Sb/CNT and Bi/CNTcomposites as anode materials for lithium-ion batteries,” MaterialsLetters 62 (2008) 2092-2095, or by J. Yan et al in “Preparation andelectrochemical properties of composites of carbon nanotubes loaded withAg and TiO₂ nanoparticle for use as anode material in lithium-ionbatteries,” Electrochimica Acta 53 (2008) 6351-6355. In this manner, theperformance of a cohesive carbon assembly-based lithium-ion batteryanode is thereby compared to the performance of lithium-ion batteryanodes composed of other materials such as graphite, hard carbon (i.e.diamond-like carbon), titanate, silicon, germanium, other CNT-basedelectrodes that require binder or structural support, and the like.

The cohesive carbon assembly of the present invention is also suitablefor use as an electrode in a fuel cell. In a PEM-type fuel cell, theelectrode comprises a catalyst support layer and a gas diffusion layer(GDL). The cohesive carbon assembly, as described earlier, has lowresistivity and high mechanical strength and integrity. Furthermore, itexhibits sufficiently high pore volume to allow the needed diffusion ofgaseous species (hydrogen, oxygen, water vapor) for fuel cell use. Thetotal pore volume of the assembly comprising SWCNT is typically greaterthan 1.0 cm³/g, often greater than 1.5 cm³/g, and has been observed toexceed 2.0 cm³/g. Total pore volume correlates with total porosity, andapproximately correlates with gas permeability. Therefore, the cohesivecarbon assembly, and in particular the SWCNT assembly, is appropriatefor use as either the catalyst support or the GDL, or as bothsimultaneously.

The size and thickness of the cohesive carbon assembly, for use in afuel cell, are not particularly limited. However, the thickness shouldbe selected such that the desired level of gas permeability ismaintained, and, when used as the catalyst layer, such that the desiredlevel of catalytic activity through the layer is achieved. The thicknessof the cohesive carbon assembly of this invention when used as acatalyst layer in a fuel cell is typically 5-20 μm thick. The thicknessof the cohesive carbon assembly of this invention when used as a GDL ina fuel cell is typically 100-300 μm thick.

For use as a catalyst support in a fuel cell, the cohesive carbonassembly is typically coated with metal particles that act as thecatalysts for the electrochemical reaction. The type of metal particlesis chosen based on whether the electrode is to be the cathode or anodein the fuel cell. For example, if the assembly is to be the anode, themetal may be platinum. If the assembly is to be the cathode, the metalmay be nickel. The coating may be accomplished by any appropriatemethod, for example, by the method described in US Patent ApplicationPublication US 2009/0015984A1. This coating method comprises twoessential steps: (1) the assembly is treated with a halogenatedprecursor, such as platinum iodide (PtI₂), nickel iodide (NiI₂),palladium iodide (PdI₂), or the like, to form a halogenatedintermediate; (2) residual halogen is removed and the metallic speciesdeposited on the assembly are reduced to pure metal by heating combinedwith hydrogen gas treatment.

To evaluate the performance of the cohesive carbon assembly as acatalyst support, GDL, or both, a PEM-type fuel cell is assembled withthe cohesive carbon assembly component in place of the standard materialtypically used for that component. For example, if the cohesive carbonassembly is the catalyst support, then it is coated with the catalystmetal particles and then installed in the fuel cell in place of thestandard catalyst support, usually Pt-coated or Ni-coated carbon black.If the cohesive carbon assembly is the GDL, then it is installed in thefuel cell in place of the standard GDL, usually carbon paper or carboncloth. If the cohesive carbon assembly is both the catalyst support andthe GDL, it is installed in place of both standard components. The fuelcell with the cohesive carbon assembly installed may be performancetested by any standard method, such as that described by B. Fang et alin “Nanostructured PtVFe catalysts: Electrocatalytic performance inproton exchange membrane fuel cells,” Electrochemistry Communications 11(2009) 1139-1141. Performance parameters such as cell voltage and powerdensity vs. current density are thus compared with those of standardfuel cells or fuel cells containing other potential alternative catalystsupport/GDL materials.

Energy storage devices such as capacitors, batteries, and fuel cells,typically comprise a current collector and an electrode on one side ofan insulating material or an electrolyte, and another current collectorand another electrode on the other side of the insulating material orelectrolyte. For example, in an electrostatic capacitor, the separatingmaterial is an insulating material, whereas in EDLCs, batteries, andfuel cells, the separating material is an electrolyte. The electrolytein and EDLC, battery, or fuel cell is divided by a thin membraneallowing ionic conduction between the electrodes. The cohesive carbonassembly of the present invention is appropriate for use as a currentcollector in these energy storage devices, due to its low resistivity,good mechanical properties, and ability to be fabricated into a desiredshape and size.

The cohesive carbon assembly may further be used concurrently as afree-standing electrode and a current collector. A free-standingelectrode, as used herein, refers to an electrode containing thecohesive carbon assembly as the only conductive material. The advantageof this is that the entire mass contributes to the usable electrodecapacity. This is in contrast to a conventional electrode where theusable electrode capacity is decreased because of mass averaging of theactive material composite layer and a metal current collector.Typically, the current collector is an aluminum or copper plate, withnotably higher mass density (2.7 and 8.8 g/cm³, respectively, for Al andCu) than that of the CNT electrode (˜0.7 g/cm³), which in turn addssignificant weight to the device.

Another advantage for free-standing electrodes is the ability to adjustthe electrode thickness that might lead to performance improvement. Forexample, in electrochemical double-layer capacitors (EDLC), thinnerelectrodes having lower resistance provide higher power density. Thisapproach to performance improvement is not feasible with conventionaldesigns due to the relative increase in the mass percent of the currentcollector.

Other advantages, more specific to the design of particular energystorage devices, are foreseeable. For example, in a battery, eliminationof the copper substrate would allow for cycling below 2.5 V (the typicalpotential where oxidation of the copper substrate initiates), thusincreasing the depth of discharge and creating the opportunity tomaintain a near-zero volt state-of-charge for prolonged storage. Ingeneral, substitution of metal current collectors with the cohesivecarbon assembly of the present invention enables entirely new designsfor these devices.

The invention is illustrated further by the following examples that arenot to be construed as limiting the invention in scope to the specificprocedures or products described therein.

EXAMPLES Example 1 Fabrication of Free Standing CNT Wafer from AqueousDispersion Using Sonication and Microfluidization Preparation of AqueousCNT Dispersion

As-received amide-functionalized SWCNT (P9 SWNT, CSI Inc., Riverside,Calif.) was ground by mortar and pestle. Three masses of the groundmaterial, 240 mg, 200 mg, and 160 mg, were separately combined with 40 geach of de-ionized (DI) water in a beaker, to create CNT/waterdispersions with ratios (mg/g) of 6:1, 5:1, and 4:1 respectively. Tofacilitate dispersion of the CNT loadings in water, each mixture wassonicated for 2 cycles of 10 minute duration (total 20 minutes) using aprobe sonicator (V600, Sonics & Materials, Inc., Newtown, Conn.). Theprobe tip was directly immersed into the CNT/water mixture forsonication. The suspension became black after 10 seconds of sonication.

Immediately after sonicating, the resulting CNT/water dispersions werepassed once through a microfluidizer (M-110Y, Microfluidics Corporation,Newton, Mass.), and collected as the effluent stream by flushing with DIwater. A stock dispersion volume of about 40 mL was collected from eachloading after one pass and used as the primary CNT/water dispersion forcasting. No precipitation, sedimentation, or phase separation wasobserved in the microfluidized dispersions, which resembled commercialconductive inks. Additional volumes of aqueous CNT dispersion, referredto as residue dispersions, were collected from the microfluidizer duringflushing with DI water to remove all trace of CNT from the instrument.

Typically, introducing 40 mL of stock CNT dispersion into themicrofluidizer (which is close to its minimum chamber fill volume)requires about four flushes with water to completely remove residual CNTfrom the instrument. These flushing steps produce an output of about 160mL of CNT dispersion volumes of varying dilution. Each of these dilutedCNT dispersions can be used for successive wafer fabrication. The first40 mL collected volumes from each prepared CNT/water ratio dispersionwere designated primary CNT dispersions, while the subsequent diluteddispersions collected by flushing were designated residual dispersionsR1, R2, R3 etc.

Preparation of CNT Assemblies

The primary microfluidized CNT/water dispersions from each loading werepartitioned into the following volumes for casting: 5 mL, 10 mL, and 15mL. Each volume was cast into a previously cleaned Pyrex dish of 5 cmdiameter and placed into a containing dish (without cover) within a fumehood. Initially, water was evaporated from the nine aqueous CNTdispersions under ambient conditions (23° C.) without any covering, asshown in FIG. 1. Unrestricted evaporation was continued until there wasa visible change in optical density and viscosity. FIG. 2 captures thesechanges as the water evaporated, comparing a dispersion from which lesswater has evaporated (A), to one from which more water has evaporated(B). When these changes to dispersions were observed, the dish anddispersion were covered with aluminum foil that was perforated with anarray of small holes using a 20G needle, to reduce the evaporation rateof the water.

Water evaporated from the CNT/water dispersions in order of their volumecapacities, that is, the 5 mL dispersions dried first, followed by the10 mL dispersions, and finally the 15 mL dispersions. After 7 days,water was completely evaporated from all but the three largest volumesof dispersion (FIG. 3(A)), and the CNT had formed into intact cohesivecarbon assemblies (wafers). At this stage, the foil coverings wereremoved from the three remaining wet dispersions and water was allowedto evaporate unrestricted in open air. After a further 5 days, thesethree dispersions also formed intact CNT wafers, with some curling (FIG.3(B)). All wafers formed intact, without cracks or fractures. The curledwafers were restored to flat by wetting them with ethanol, then pressingthem between glass plates and allowing to dry at room temperature. Afterthis procedure, the wafers remained flat and did not curl again (FIG.4).

Wafers produced from 5 mL dispersions had final thicknesses betweenabout 15 and 20 μm, those produced from 10 mL dispersions had finalthicknesses between about 25 and 45 μm, and those produced from 15 mLdispersions had final thicknesses between about 45 and 60 μm. Waferthickness also roughly correlated with the total amount of CNT (and withthe CNT:area ratio), especially when the ratio was between about 1 and 3mg CNT per cm² of container area.

Example 2 Fabrication of Free Standing CNT Wafer from Aqueous DispersionUsing Refluxing and Microfluidization Preparation of Aqueous CNTDispersion

Approximately 400 mg of carboxyl-functionalized SWCNT (P3 SWNT, CSIInc.) were combined with 80 mL of DI water in a round bottom flask tocreate a CNT/water dispersion with ratio of 5:1 (mg/g). The CNT/watermixture was refluxed in boiling water while stiffing for 4 hours. Theresulting dispersion was divided into two 40-mL portions, and each ofthese was passed through the microfluidizer one time. For each 40 mLportion, the primary and three residual dispersions (R1-R3) werecollected as the effluent streams by flushing with DI water as describedin Example 1. CNT concentrations in the residual dispersions weredetermined by casting fixed volumes of each into dishes and thenweighing the resulting assemblies after completely evaporating thewater. The CNT concentrations of dispersions R1, R2, and R3 weredetermined to be 2.0, 0.8, and 0.4 mg/g, respectively.

Preparation of CNT Assemblies

One of the two 40 mL primary CNT/water dispersions was cast into a Pyrexdish of 15 cm diameter, and covered with perforated Al foil. The waterwas allowed to evaporate under ambient conditions, and a dry, monolithicCNT wafer formed after 9 days. Some discontinuities were present inwafer, but it was completely cohesive and intact otherwise. Thethickness was 22 μm. This CNT wafer is shown in FIG. 5(A).

The second 40 mL primary CNT/water dispersion was partitioned into four5 mL portions and one 15 mL portion. The 15 mL portion was cast into a 9cm Pyrex dish, and two of the 5 mL portions were cast into two 5 cmPyrex dishes. The dishes were covered with perforated Al foil and thewater was evaporated under ambient conditions. FIGS. 5(B) and 5(C) showresulting dry, intact, cohesive 9-cm and 5-cm CNT wafers that formed uafter 6 days. The thickness of the 9-cm wafer was 23 μm. The thicknessof the 5-cm wafers was between 20 and 25 μm.

Example 3 Fabrication of Adherent CNT Assemblies on Aluminum Substrates

Aqueous CNT (CSI Inc. P3 SWNT) dispersions were prepared via thesonication-microfluidization route (Example 1) and therefluxing-microfluidization route (Example 2).

For the aqueous CNT dispersion prepared according to Example 1, residualdispersions R1, R2, and R3 were collected after microfluidization, asdescribed in Example 2. About 5 ml of the first (R1) residual dispersionwere cast directly into an aluminum foil pan 4 cm in diameter, which hadbeen etched with dilute NaOH solution for 2 minutes to promote adhesionof the CNT assembly. The etched Al pan was lined along its insideperiphery with tape to expose only the base for CNT contact andadhesion. To another similarly treated Al pan, 5 mL of R2 CNT/waterdispersion were cast. Water was allowed to evaporate from the castdispersions under ambient conditions of temperature and pressure.Evaporation completed in about three days.

For the aqueous CNT dispersion prepared according to Example 2, about 5ml each of the primary dispersion were cast into two NaOH-etchedaluminum foil pans, and water was evaporated under ambient conditions oftemperature and pressure. Evaporation completed in about three days.

After evaporating the water, the cast dispersions all formed into intactCNT assemblies that adhered to the aluminum pans. The CNT assemblies hadno cracks, fractures, or other visible discontinuities, as shown in FIG.6(A). Shapes such as squares were cut from the assemblies for testing,as shown in FIG. 6(B). The CNT assemblies were about 15 μm thick.

Example 4 Fabrication of Adherent CNT Assemblies on Polymer Substrates

An aqueous CNT dispersion was prepared via thesonication-microfluidization route described in Example 1. The residualdispersions R1, R2, and R3 were collected after microfluidization asdescribed in Example 2. About 5 mL of the R1 dispersion were cast into a5-cm diameter Pyrex dish into which had been placed a circular insert ofpolyethylene terephthalate (PET) film. Water was evaporated from thedispersion under ambient conditions and was completely removed afterthree days. The CNT formed into an intact assembly which partiallyadhered to the PET film. The adherent portions were flexible and did notcrack, fracture, or flake when the film was flexed. A ⅝-inch disk wascut from the adherent portion of the CNT/PET film structure and remainedintact and flexible (FIG. 7). The thickness of the CNT assembly was 17μm.

Example 5 Continuous Fabrication of CNT Assemblies

A schematic diagram of an apparatus for continuous CNT wafer fabricationis shown in FIG. 8.

An aqueous CNT dispersion is prepared according to the method describedin Example 2. The CNT/water dispersion is continuously added to a tankor hopper. From the tank or hopper, the dispersion is continuouslydispensed at a controlled rate onto a conveyor belt that is moving at acontrolled speed. The dispersion feed rate and belt speed are controlledto produce a CNT assembly of desired thickness.

To produce a CNT assembly on a substrate (such as polymer film or metalfoil), the conveyor belt feeds the desired substrate material withchosen thickness, size, and shape. After the dispersion is fed onto thesubstrate, the belt moves the dispersion and substrate through anevaporation area at a controlled rate. Water is evaporated at ambientroom temperature and pressure with optional heating to increase dryingrate. The CNT form into an assembly which is adherent on the substrate,and emerges from the evaporation area as a strip or ribbon. The assemblyis then continuously collected by spooling. Optionally, the assembly isdirected to a cutter to produce a desired shape and size of individualassemblies.

To produce a continuous, free-standing CNT assembly, the conveyor beltsurface material is selected or treated such that it is sufficientlyhydrophobic to prevent the CNT assembly from adhering to it. Optionally,the conveyor belt has a groove or depression parallel to the directionof motion, into which the dispersion is dispensed. Water is evaporatedfrom the dispensed dispersion as above and the CNT assembly forms,emerging as a free-standing strip or ribbon, which is collected by aspool. Optionally, the assembly is directed to a cutting apparatus toproduce a desired shape and size of individual assemblies.

Example 6 Coherent SWCNT Coating Films on Substrates (Spin-Coated)

One (1) ml of the aqueous CNT dispersion with CNT:water ratio of 6:1(mg:g), prepared according to Example 1, is spin-coated on a siliconwafer substrate at 500 rpm for 30 sec. Water evaporates from thedispersion, and the CNT forms into a coating film on the substrate, witha thickness of about 0.5 μm.

Example 7 Coherent SWCNT Coating Films on Substrates (Dip-Coated)

Approximately 1 liter of the aqueous CNT dispersion with CNT:water ratioof 6:1 (mg:g), prepared according to Example 1, is placed in a tallrectangular plastic tank (30×12×3 cm). 4×6 inch sheets of copper andaluminum, and a 4-inch diameter silicon wafer are sequentially dippedinto the dispersion, and each substrate is withdrawn from the dispersionat a speed of 0.2 cm/s. Water is evaporated from each dipped sampleunder ambient conditions. The CNT forms into a cohesive coating film oneach substrate, with a thickness of about 5 μm.

Example 8 Coherent SWCNT Coating Films on Substrates (Spray-Coated)

The aqueous CNT dispersion prepared according to Example 1 withCNT:water ratio of 6:1 (mg:g), is sprayed on an aluminum foil substrateand the water is evaporated under ambient conditions. The CNT forms intoa cohesive coating film on the substrate with a thickness of about 50μm.

Example 9 Fabrication of Transparent CNT Assemblies

An aqueous CNT dispersion was prepared via thesonication-microfluidization route described in Example 1. The residualdispersions R1, R2, and R3 were collected after microfluidization asdescribed in Example 2. About 5 mL of the R3 dispersion (the mostdiluted CNT/water dispersion collected from the microfluidizer) werecast into a 5-cm diameter glass dish. Water was evaporated from thedispersions under ambient conditions without restriction for 3 days.

After evaporation of the water, the CNT formed into continuous,cohesive, thin wafer segments which were removed from the glass dish asfree standing, transparent CNT wafer portions about 1 cm in length. Ahigh resolution electron microscope image of a portion of thetransparent cohesive CNT wafer over a TEM copper grid is shown in FIG.9. The thickness of the transparent CNT assembly was 120 nm.

Example 10 Fabrication of Free Standing CNT Wafer from AqueousDispersion Using Sonication

Approximately 50 mg of Baytubes® MWCNT (Bayer MaterialScience,Leverkusen, Germany, containing approximately 0.1-0.5 at. % of (—COOH)and/or (—OH) functional groups, was introduced into 25 mL of water in abeaker and processed by probe sonication using 3 cycles of 15 minuteduration. The resulting CNT/water dispersion was prepared withapproximate ratio of 2:1 (mg/g).

About 5 mL of the CNT/water dispersion was cast into a Pyrex dish of 3cm diameter. Water was evaporated from the dispersion in air at roomtemperature without covering the dish. After 16 hours, the water wascompletely evaporated and the CNT formed into an intact and cohesivefree standing CNT wafer. FIG. 10 shows the resulting intact, cohesive3-cm CNT wafer that formed upon removal of the water. The thickness ofthis wafer was about 10 μm.

Example 11 Fabrication of Adherent CNT Wafer from Aqueous DispersionUsing High Shear Mixing and Sonication

Approximately 300 mg of Baytubes® MWCNT were introduced into 200 mL ofwater in a sealed flask. The CNT/water mixture was dispersed using ahigh shear mixer (10,000 rpm for 60 min) while simultaneously sonicatingin an ultrasonic bath for 60 min. The dispersion was then sonicated foran additional hour at 45° C. using the ultrasonic bath. The resultingCNT/water dispersion was prepared with approximate ratio of 1.5:1(mg/g).

About 50 mL of the CNT/water dispersion was cast into a silicone rubberretainer of approximate exposure dimensions 15 cm×3.5 cm placed overaluminum foil (All foils, 25 thick). The silicone retainer formed aleak-tight seal with the aluminum foil. Water was evaporated from thedispersion at room temperature and was completely evaporated after 1day. The CNT formed into an intact assembly which adhered to thealuminum foil (FIG. 11). The thickness of the CNT assembly was about 25μm.

Example 12 Fabrication of mm-thick CNT assembly from Aqueous DispersionUsing Sonication

Approximately 120 mg of Baytubes® MWCNT were introduced into 40 mL ofwater in a beaker and processed by probe sonication using 3 cycles of 15minute duration. The resulting CNT/water dispersion was prepared withapproximate ratio of 3:1 (mg/g).

The CNT/water dispersion (40 mL) was cast into a 50 mL capacity glassbeaker and the water was evaporated overnight (or for about 16 hours) ina convection oven at 40° C. The CNT formed into a free-standing cohesivedisc-shaped structure as shown in FIG. 12. The cohesive MWCNT disc wasabout 3-4 mm thick with a mass of about 500 mg. The MWCNTmacro-structure possessed a matte top surface and shiny bottom surface.

Example 13 Electrical Resistance and Resistivity of CNT Assemblies

To be useful as a current collector for such devices as a capacitor,fuel cell, or battery, a material needs to have sufficiently lowresistivity (on the order of 10⁻² Ω-cm or below) and sufficientmechanical robustness (high tensile strength and resistance tobreakage).

To establish that the cohesive assemblies have sufficiently lowresistivity to be used as current collectors, assemblies prepared asdescribed in Example 1 were measured for electrical sheet resistance andresistivity as follows:

From each assembly, a sample of rectangular or square geometry was cutwith lengths greater than 1 cm on all sides. Each sample was mounted ina sample mount, and two electrical contact pairs (two current carryingand two voltage sensing) were directly compressed to the sample, in astandard Kelvin-type (4-point) probe configuration. The sample waspositioned such that the four metal tips of the four-point probe madedirect contact with the sample without puncturing through it.

A constant current of 1 mA was made to flow the length of the sample byusing a high impedance current source. The voltage drop across thesample was measured using a high impedance digital voltmeter. Thesurface (sheet) resistance, R_(s) in Ω (or Ω/sq), of the sample wasdetermined from the ratio of the stable voltage registering on thevoltmeter, V, to the value of the output current of the current source,I, multiplied by the geometric factor π/ln2≈4.53:

R _(s)=4.53 (V/I).

The thickness (t) of each sample was measured using a profilometer,digital micrometer, or scanning electron microscope, and the electricalresistivity ρ of each sample in Ω-cm, was then calculated using theformula:

ρ=R _(s)(t)

Sheet resistances of SWCNT assemblies prepared according to Example 1were between 27 and 41Ω per square. Resistivities of cohesive carbonassemblies prepared according to Example 1 were between about 0.06 and0.22 Ω-cm.

Resistivities of some of the SWCNT assemblies were sufficiently low suchthat they could be utilized as current collectors in electronic storagedevices such as capacitors, fuel cells, or batteries. Moreover, cohesiveassemblies fabricated per the present invention possess the necessarymechanical properties to be used as current collectors, replacingcurrent collectors made from metals such as aluminum or copper. This isin direct contrast to other types of carbon assemblies, including, forexample, activated carbon, and other CNT-based assemblies such as thosemade from CNT forests, which do not possess the necessary robustness tobe used as current collectors in place of metal plates.

Example 14 Capacitor Electrode Comprising a Cohesive Assembly of SWCNT

Cohesive carbon assemblies are prepared following the proceduredescribed in Example 2. The SWCNT assemblies are about 9 cm in diameterand about 20-35 μm thick (measured using a profilometer, model Dektak150, Veeco Instruments Inc., Plainview, N.Y.). Discs about 0.625 inch indiameter are cut from the assemblies using a standard laboratory blade.

Some of the discs are placed in a sealed quartz tube inside a furnace atroom temperature (about 20° C.). The tube is purged for one hour byflowing helium through it at 20 sccm. The discs are then heated in thefurnace at 10° C./minute to 1000° C. while continuing the flow ofhelium. While holding the temperature at 1000° C., helium flow isstopped, and a mixture of 5% chlorine and 95% argon gas is introduced at20 sccm. These conditions are maintained for 1 hour, then the gas isswitched back to helium at 20 sccm for 30 minutes. The gas is thenchanged to a mixture of 5% hydrogen and 95% argon at 20 sccm for 30minutes to remove residual chlorine. Then, the gas is switched back to20 sccm helium and maintained for 2 hours. The furnace is then coolednaturally to room temperature.

The discs treated with chlorine as above, and some non-treated discs,are then dried under vacuum at 195° C. for 12 hours immediately prior tofurther use.

For comparison with the SWCNT discs, Activated Carbon (AC) with theproduct name Norit DLC Super 30 is obtained from Norit Nederland BV(Amersfoort, The Netherlands). A disc-shaped piece about 0.625 inch indiameter and between 40 and 60 μm thick is formed from the AC powderusing standard manufacturing methods. The AC disc is dried at 60° C. for1 hour immediately prior to further use.

Electrochemical double-layer capacitor (EDLC) cells are fabricated usingthe SWCNT and AC discs. Prototype cells are assembled in a dry box usingmetal plates clamped against each electrode face as current collectors.The cells are tested for their properties and performance as electrodesin symmetric electrochemical capacitors rated at 2.0 volts, using 1.0Mtetraethylammonium tetrafluoroborate (TEATFB) salt in propylenecarbonate as the electrolyte.

Test capacitor cells are conditioned by holding them at 2.0 V for tenminutes, then charge/discharge cycled using a battery/capacitor tester(Model BT2000, Arbin Instruments, College Station, Tex.) thirty timesbetween 1.0 and 2.0 V using 2.5 mA current. Then, electrical performancemeasurements are made in the following order:

-   -   1. Leakage current after 30 minutes at 1.0, 1.5, and 2.0 V    -   2. EIS (electrochemical impedance spectroscopy) measurements at        2.0 V bias voltage    -   3. Constant-current and constant-power charge/discharge        measurements using the Arbin tester

Representative results of the above measurements are summarized in Table1.

Equivalent series resistances (ESR) of all cells were comparable.

The 30 minute leakage current of cells fabricated with electrodes madefrom both non-treated and Cl₂-treated SWCNT assemblies is superior (i.e.lower) compared to that of the cell fabricated with AC electrodes.Chlorine-treated SWCNT cells show slightly better leakage currentcompared to non-C1₂-treated cells, but the difference might bestatistically insignificant. The much lower leakage current of the SWCNTcells suggests that they may be operated at substantially highervoltages compared to AC-based cells.

The SWCNT cells exhibit very high discharge rate, with full capacitancedischarge times on the order of about 0.3-0.4 seconds, or less, which isretained up to 5 A/g current, compared to about 2-3 seconds for the ACcell. Furthermore, the AC cell does not retain full capacitance to thissame current level. This indicates that the SWCNT cells show superiorpower performance compared to the AC cell.

Power density of the SWCNT cells is estimated to be at least 100 kW/kg.This is superior to the power density of typical commercial AC-basedEDLCs, which is about 10 kW/kg or less, and is at least equivalent tothe power performance of any currently available commercial EDLC.

Due to limitations within the test, absolute full capacitance dischargetime and power density of the SWCNT cells can not be determined. Forthese two parameters, the SWCNT cells show power performance in excessof the measurement capability of the test equipment.

Another strong indicator of pulse power performance of a capacitordevice is the frequency at which the complex impedance phase anglereaches 45°. A higher frequency indicates better performance. Capacitorsfabricated from SWCNT electrodes show 45° phase angle frequency of about2.4 Hz, whereas the capacitor based on AC shows 45° phase anglefrequency of 0.5 Hz. For this performance metric, Cl₂-treated andnon-treated SWCNT cells perform similarly.

Overall, SWCNT electrodes of the present invention out-performcommercial activated carbon currently used as the standard electrodematerial in EDLC devices, in terms of pulse power performance.

TABLE 1 Performance of Capacitor Cells Utilizing SWCNT Electrodes of thePresent Invention, Compared to Activated Carbon Electrode. EIS Fre- Cell30 minute leakage Power quency for ID and ESR current (μA) DischargeDensity 45° phase Type (Ω) 1.0 V 1.5 V 2.0 V Time (sec) (kW/kg) angle(Hz) No Cl₂ 0.64 1.6 2.4 5.0 0.3-0.4* >100^(#) 2.4 Cl₂ 0.50 0.7 1.4 3.82.4 treated AC 0.54 3.9 7.3 33.1 ~2-3  ~10  0.4-0.5 *Minimum measurablefull capacitance discharge time under the test conditions; actual timeis lower. ^(#)Maximum measurable power density under the testconditions; actual power density is higher.

Example 15 Battery Electrode Comprising a Cohesive Assembly of SWCNT

A cohesive carbon assembly is prepared following the procedure describedin Example 2. The SWCNT assembly is about 9 cm in diameter and about25-35 μm thick.

A section of appropriate size and shape is cut from the assembly andtested for its performance as an anode in a lithium-ion battery, usingthe method described by Y. NuLi in Materials Letters 62 (2008)2092-2095.

The test method consists of the following essential steps: (1) thecohesive SWCNT assembly is installed in a test battery cell, (2) thecell is discharged, and (3) the power and energy densities from thedischarge curves are measured.

Then, the data for the cell with the SWCNT anode is compared with thesame data obtained from a sampling of similar cells having other typesof anode materials. The performance of the SWCNT assembly-basedlithium-ion battery anode is thereby compared to the performance oflithium-ion battery anodes composed of other materials such as graphite,hard carbon (i.e. diamond-like carbon), titanate, silicon, germanium,other CNT-based electrodes that require binder or structural support,and the like.

Example 16 Fuel Cell Electrode Comprising a Cohesive Assembly of SWCNT

A cohesive carbon assembly is prepared following the procedure describedin Example 2. The SWCNT assembly is about 9 cm in diameter and about25-35 μm thick (measured by profilometer).

A piece of the SWCNT assembly is analyzed by nitrogenadsorption/desorption using a model TriStar 3000 equipment manufacturedby Micromeritics Instrument Corp., Norcross, Ga. The assembly has a BETsurface area of 1680 m²/g, and a total desorption pore volume of 1.75cm³/g. The density of the assembly is determined to be about 0.5 g/cm³by dimensional and weight measurements. The porosity of the assembly isthereby calculated as about 88%.

A section of appropriate size and shape is cut from the assembly and thesection is then coated with platinum metal particles according to amethod described in US Patent Application Publication US 2009/0015984A1.

The Pt-coated section of SWCNT assembly is evaluated for its performanceas a fuel cell electrode, using the method described by B. Fang et al,Electrochemistry Communications 11 (2009) 1139-1141. The cell voltageand power density vs. current density behavior of the fuel cellcontaining the SWCNT assembly-based electrode, is then compared to theperformance of standard fuel cells containing carbon black, carbonpaper, and/or carbon cloth-based electrodes, and to the performance offuel cells containing other potential alternative electrode materials.

Although several embodiments of the invention have been described in theExamples given above, those of ordinary skill in the art will appreciatethat various modifications can be made without departing from the scopeof the invention. Accordingly, other embodiments are within the scope ofthe following claims.

1. A method of preparing a cohesive carbon assembly comprising: a.obtaining a functionalized carbon starting material in a form of powder,particles, flakes, loose agglomerates, aqueous wet cake, or aqueousslurry; said functionalized carbon starting material is a carbonstarting material which has been covalently or non-covalentlyfunctionalized such that it is dispersible in water; b. dispersing thefunctionalized carbon starting material in an aqueous solution in aprescribed ratio to form a dispersion; and c. substantially removingliquid from the dispersion in a controlled manner, whereby the cohesivecarbon assembly is formed.
 2. The method of claim 1, wherein the carbonstarting material is selected from the group consisting of carbonnanotubes, graphene, graphene oxide, graphite, expanded graphite,exfoliated graphite, amorphous carbon, activated carbon, and anycombinations thereof.
 3. The method of claim 2, wherein thefunctionalized carbon starting material contains acyl chloride,carboxyl, hydroxyl, amide, glycerol, organic ester, PEO, PEG, PVA, aminoterminated polystyrene, BSA, PABS, or any combination thereof.
 4. Themethod of claim 2, wherein the carbon starting material is carbonnanotubes.
 5. The method of claim 4, wherein the carbon nanotubes aresingle-walled carbon nanotubes, double-walled carbon nanotubes,multi-walled carbon nanotubes, or any combination thereof.
 6. The methodof claim 1, wherein the aqueous solution is de-ionized water, distilledwater, purified water, or any combination thereof.
 7. The method ofclaim 1, wherein step b further comprises dispersing the carbon startingmaterial in water in the presence of mechanical agitation.
 8. The methodof claim 7, wherein the mechanical agitation comprises sonication,mechanical stirring, high shear mixing, microfluidization,homogenization, or any combination thereof.
 9. The method of claim 1,wherein the carbon starting material is functionalized single-walledcarbon nanotubes, the water is purified and de-ionized water, and theratio of carbon starting material and water is between about 0.1 andabout 20 mg carbon per gram of water.
 10. The method of claim 1, whereinthe carbon starting material is dispersed in the aqueous solution thatis substantially free of a binding material.
 11. The method of claim 1,further comprising applying the dispersion to a hydrophobic surfaceafter step b and before step c.
 12. The method of claim 11, wherein thehydrophobic surface comprises a dimethyl organosilane, a fluorinateddimethyl organosilane, a fluorinated polymer or Teflon.
 13. The methodof claim 11, wherein the dispersion is applied to the surface bycasting.
 14. The method of claim 1, further comprising applying thedispersion to a hydrophilic surface after step b and before step c. 15.The method of claim 14, wherein the hydrophilic surface is selected fromthe group consisting of metal surface, glass surface, silicon surface,plastic, ceramic and any combination thereof.
 16. The method of claim15, wherein the metal is selected from the group consisting of aluminum,copper, gold, silver, platinum, tantalum, titanium, stainless steel, andany combination thereof.
 17. The method of claim 14, wherein thedispersion is applied to the surface by spin-coating, dip-coating,flow-coating, spray coating, casting, or any combination thereof.
 18. Acohesive carbon assembly prepared by the method of claim
 1. 19. Thecohesive carbon assembly of claim 18, wherein the carbon startingmaterial comprises functionalized carbon nanotubes.
 20. The cohesivecarbon assembly of claim 18, having an electrical resistivity of lessthan about 0.2 Ω-cm.
 21. The cohesive carbon assembly of claim 18,having an electrical sheet resistance of less than about 50Ω per square.22. The cohesive carbon assembly of claim 18, wherein the assembly istransparent.
 23. A capacitor comprising two capacitor electrodesseparated by an insulating material, wherein at least one of the twocapacitor electrodes comprises the cohesive carbon assembly of claim 18.24. A fuel cell comprising two electrodes separated by an electrolyte,wherein at least one of the two electrodes comprises the cohesive carbonassembly of claim
 18. 25. A battery comprising two electrodes separatedby an electrolyte, wherein at least one of the two electrodes comprisesthe cohesive carbon assembly of claim
 18. 26. An energy storage devicecomprising a current collector and an electrode on one side of aninsulating material, and another current collector and another electrodeon the other side of the insulating material, wherein at least one ofthe two current collectors comprises the cohesive carbon assembly ofclaim
 18. 27. An energy storage device comprising a current collectorand an electrode on one side of an electrolyte, and another currentcollector and another electrode on the other side of the electrolyte,wherein at least one of the two current collectors comprises thecohesive carbon assembly of claim
 18. 28. An article comprising asubstrate and a cohesive carbon assembly coated onto at least onesurface of the substrate, wherein the cohesive carbon assembly has beenprepared by the method of claim 14.